Sandwich-type acoustic material in a flexible sheet form

ABSTRACT

A SANDWICH-TYPE, ELECTRO-MECHANICAL, ABSORBING, ACOUSTIC MATERIAL, IN A FLEXIBLE SHEET FORM, COMPRISING A THIN SHEET OF ELASTOMERIC MATERIAL WITHIN WHICH ARE EMBEDDED PATICLES HAVING A SIZE BARELY GREATER THAN THE THICKNESS OF THE ELASTOMERIC MATERIAL. THE PARTICLES MAY BE OF A TRANSDUCER MATERIAL, GENERALLY SINGLE-CRYSTAL, AND/OR OF A   RESISTIVE MATERIAL. TWO CONDUCTIVE SHEETS, ONE ON EACH SIDE OF THE ELASTIMERIC MATERIAL, WHICH MAKE CONTACT WITH THE EMBEDDED PARTICLES COMPLETE THE SANDWICH.

Oct. 26, 1971 WHITEHQUSE E.'I'AL 3,614,992

SANDWICH-TYPE ACOUSTIC MATERIAL IN A FLEXIBLE SHEET FORM Filed May 26, 1969 INVENTORS.

HARPER JOHN WHITEHOUSE SHELBY F. SULLIVAN BY ERVIN F. JOHNSTON ATTORNEY.

JOHN STAN, AGENT.

nited States Patent 3,614,992 SANDWICH-TYPE ACOUSTIC MATERIAL IN A FLEXIBLE SHEET FORM Harper John Whitehonse, Hacienda Heights, and Shelby F. Sullivan, Arcadia, Calif., assignors to the United States of America as represented by the Secretary of the Navy Filed May 26, 1969, Ser. No. 827,578 Int. Cl. G101: 11/04; E04b 1/84 US. Cl. Isl-33 G 8 Claims ABSTRACT OF THE DISCLOSURE The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

In the prior art, acoustic materials of this type were formed either by the viscous loss of a liquid flowing through multiple orifices or by the absorption of a gas film absorbed on aluminum flake dispersed in a rubber matrix. While the first technique is pressure-independent, only a modest absorption may be achieved since increasing the viscosity of the liquid does not increase the absorption above certain limits because of compensating effects in the liquid. In the second method, the absorption is large, but decreases rapidly with increasing hydrostatic pressure due to collapse of the adhered gas film, thus not providing a material suitable for deep submergence, for which environment this type of acoustic material is particularly useful.

The sandwich-type material of this invention provides an absorption mechanism independent of depth because the piezoelectric properties of the material do not depend on the hydrostatic pressure to which the individual crystals of the particulate material are subjected. With increasing pressure applied to a piezoelectric material, the direct current output voltage across the material increases, but for a time-varying acoustic signal across the piezoelectric material this is not true, since the magnitude of the output signal does not depend on what is effectively a bias signal.

Ferroelectric materials, which can also be used for the purposes of this invention, are similar to piezoelectric materials in that they have similar transduction properties when they are below the Curie temperature, which would generally be the case for underwater use.

Not all transducer materials are useful for the purposes of this invention, but only those materials which produce electrical energy in response to a time-varying hydrostatic pressure.

The sandwiched acoustic material of this invention is formed between two thin conductive films, such as aluminum film. Between the films is a distribution of pressure-sensitive, electro-mechanical, transducer particles, for example, ferroelectric or piezoelectric particles, generally single-crystal particles, in a binding matrix of plastic. Resistive particles also may be used, in any desired proportion to provide required energy dissipation. The particles have an average size on the order of 10 p CC microns. The layer of particles is controlled to be only one particle thick. The thickness of the super-thin elastomeric web material is on the same order of size, or somewhat smaller.

One of the primary advantages of the material of this invention is that absorption is essentially independent of pressure. A second advantage is that the material may be used as a filler sheet in composite structures, providing arbitrary thickness and a capability of being formed into intricate shapes and around arbitrarily shaped bodies. The combination of active transducers and dissipative material in a single sheet permits precise compositional control. The fabrication of a sheet-like material rather than a three-dimensional matrix which must be cast or machined for each different application is another advantage of the new acoustic material.

It is well known in the art that the lower the frequency at which a desired amount of absorption of acoustic energy is required, the greater the thickness of absorbing material that is needed, and, hence, the greater the number of layers of the sandwich-type material which will be required.

The sandwich-type material may be used to form multiple layers about some other material, the number of layers being determined by the total amount of absorption required of the material. The material may be used, for example, as the backing for hydrophone arrays, in order to improve the performance of the arrays. The acoustic waves incident upon the arrays backed with the material of this invention would not suffer from spurious unwanted reflections caused by the mechanical structure which is supporting the absorbing material and the arrays.

Accordingly, an object of the invention is the provision of an acoustic material in which absorption of acoustic energy is essentially independent of pressure.

Another object of the invention is the provision of an acoustic material whose absorptivity is not dependent upon the viscosity of any liquid or on the absorptive capability of any gas used in the process of making the material.

Still another object of the invention is the provision of an acoustic material which is much simpler to construct than similar materials in the prior art.

A further object of the invention is the provision of an acoustic material having any desired amount of energy dissipation.

Yet another object of the invention is the provision of an acoustic material capable of being formed into intricate shapes.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description, when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIG. 1 is a three-dimensional view of the sandwiched acoustic material of this invention containing only one type of particulate matter.

FIG. 2 is an end view of the sandwiched acoustic material of this invention containing two types of particulate matter.

The commercially available technology for forming these super-thin sandwiches was developed by Mr. T. S. te Velde of the International Philips Co., and is described on p. 83 of the December 1967 issue of IEEE Spectrum, and is summarized immediately following.

The process for making the thin, sandwich-type, acoustic material comprises the steps of: applying a thin layer of glue to a substrate; sprinkling powdered particles upon the glue; permitting the glue to harden; removing the excess particles; dipping the substrate with its glue in an elastomer for a suflicient length of time so that the tops of the particles barely protrude from the elastomeric material; permitting the elastomer to harden; removing the substrate and glue; and evaporating a metallic film onto each side of the remaining acoustic material.

Discussing now the figures in more detail, and beginning with FIG. 1, therein is shown a sandwich-type material comprising: a sheet of thin elastomeric material 12; particulate material 14, embedded in the elastomeric material 12, having a size slightly greater than the thickness of the elastomeric material, the individual particles belonging to one or the other of the following categories: (1) transducer particles, generally of a single crystal; (2) resistive particles. Two conductive sheets 16, one on each side of, and making physical contact with, the particulate material 14 complete the sandwich-type structure.

The elastomeric material 12 may be polyurethane, while the particulate material 14 may be transducer particles of antimony sulpho-iodide.

The two conductive sheets 16 may be of aluminum, either evaported onto the combination of antimony sulpho-diodide particles 14 and polyurethane 12, or may be sheets glued to the combination by a conductive adhesive, for example a conductive epoxy. The conductive coatings 16 connect all particles together so that they are electrically parallel. Antimony sulpho-iodide as the active transducer material 14 is simultaneously ferroelectric below 23 C. and semiconducting. The voltage produced by the acoustic wave interacting with a particle cell dissipates as heat in the combined parallel resistance of the other particles, which are all connected in parallel electrically by the aluminum films 16.

Barium titanate and lead titanate zirconite, etc., could be used in place of the single-crystal ferroelectric antimony sulpho-iodide as the active transducer element 14.

Discussing now the embodiment shown in FIG. 2, this is similar to the sandwich-type acoustic material shown in FIG. 1, except that in this instance the particulate matter consists of active, nonconductive, transducer particles 14, which may be piezoelectrics such as lithium sulfate monohydrate, tartaric acid and tourmaline, and electrically passive, dissipative, that is, resistive, particles, for example carbon granules 22. The proportion of carbon granules can be so chosen as to give any desired energy dissipation.

Efiectively, the particulate material acts as a generator producing electrical energy which is dissipated in a load, the transducer particles converting incident acoustic energy into generated electrical energy which is dissipated in resistive particles acting as a load. In the case of antimony sulpho-iodide particles, since each particle is simultaneously a transducer and semiconductor, which by definition has resistance, the energy is both generated and dissipated in the same particles and no other, different, resistive particles are needed.

The particulate material is not energy-absorbing without the presence of some kind of a resistive load connected across the two conductive sheets 16.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An acoustically absorbing material comprising:

a sheet of thin elastomeric material;

particulate material having a dimension slightly greater than the thickness of the elastomeric material and being embedded therein so that each particle protrudes from opposite surfaces of the elastomeric material;

the particulate material including transducer particles of the type that produces electrical energy in response to hydrostatic pressure;

a pair of flexible conductive sheets; and

the combination of elastomeric and particulate materials being sandwiched between the pair of flexible conductive sheets, with the particulate material making electrical contact with both of the conductive sheets.

2. A sandwich-type acoustic material according to claim 1, wherein the transducer particles are singlecrystal transducer particles.

3. A sandwich-type acoustic material according to claim 1, wherein the elastomeric material is polyurethane.

4. A sandwich-type acoustic material according to calim 2, wherein the two conductive sheets are of aluminum.

5. A sandwich-type acoustic material according to claim 3, wherein:

the transducer particles are of antimony sulpho-iodide.

6. A sandwich-type acoustic material according to claim 2, wherein:

the particulate material further comprises resistive particles.

7. A sandwich-type acoustic material according to claim 6, wherein:

the elastomeric material is polyurethane;

the transducer particles are of tourmaline; and

the resistive particles are carbon particles.

8. A process for making a sandwich-type, acoustic material in a flexible sheet form which comprises the steps of:

applying a thin layer of glue to a substrate;

sprinkling powdered transducer particles of a type that produce electrical energy in response to a hydrostatic pressure upon the glue;

permitting the glue to harden;

removing the excess particles;

dipping the substrate with its glue in an elastomer for a sutficient length of time so that the tops of the transducer particles barely protrude from the elastomeric material;

permitting the elastomer to harden;

removing the substrate and glue; and

evaporating a metallic film onto each side of the remaining acoustic material.

References Cited UNITED STATES PATENTS 2,332,860 7/1967 Diebold et al. 2O438 3,392,056 7/1968 Maskalick 1l7227 3,424,270 1/ 1969 Hartman et al. 18l-33 3,514,326 5/1970 Stow 117-227 OTHER REFERENCES Publication IEEE Spectrum, December 1967, p. 83.

STEPHEN J. TOMSKY, Primary Examiner US. Cl. X.R. 117107, 227 

